Discharge lamps of different forms have been in use for about a century. Today, gas discharge lamps, such as mercury vapor, sodium vapor and metal halide lamps, continue to be a large segment of the lighting industry. Typically, the lamps are formed from a continuous sealed vessel which holds the vapor or gas, which is electrically excited by a voltage application between spaced apart metal conductor electrodes.
For a given voltage to pressure ratio in a hollow cathode gas discharge lamp, it is well known that the emitted light intensity increases as a function of the pressure within the vessel. However, the pressure is limited by the strength of the vessel material, which must be transparent or translucent to create an effective light source. Another limitation is the diameter of the cathode formed in a traditional manner, typically in metals, which sets a second pressure limitation on operation as a true hollow cathode discharge.
A consideration in displays is resolution, typically defined by pixels per inch. Conventional hollow cathode discharge lamps are formed in metals. In such conventional lamps, currently utilized machining techniques for metals have limited discharge cavity diameters to about 700 .mu.m or more. Though a #80 drill bit, having a diameter of about 338 .mu.m, may be used to drill very shallow holes in metals, aspect ratios for hollow cathode operation (about 4:1 depth to aperture) have not been obtained. The potential resolution of a display is accordingly limited to a pixel size which exceeds 700 .mu.m. The need to combine separate discharge effects for color variation further decreases potential resolution.
One solution to these problems is proposed in U.S. Pat. No. 5,438,343 to Khan et al. That patent contemplates a large number of microcavities, each of which permits a higher pressure than a single large cavity. The microcavities are formed via wafer bonding of two micromachined substrates of fused quartz, sapphire, glass or other transparent or translucent material. Cavities in the separate substrates align to form vessels for containing filler after the substrates are bonded. While a RF "electrodeless" embodiment is disclosed, other embodiments include etched recesses adjacent the vessels in one or both of the substrates for accommodating separate metal electrodes. After the electrodes are deposited or otherwise placed in the recesses to electrically contact the filler, the separate substrates are bonded together through Van der Waal's forces.
Separate plugs are required at the point where the electrodes are located to maintain the vacuum integrity of the vessel. The plug material, which may be glass, is deposited over the metal layer to strengthen the microcavity which forms the filler vessel. This additional step is required to reinforce the cavity, which is weakened by the recess necessary to accommodate a separate electrode. Together, the reliance on Van der Waal's forces to bond separate substrates and the need for reinforcing plugs limit the potential pressure within the vessel, and significantly complicate production of the device. Pressure is also limited by formation of striations in the discharge and arcing. Another difficulty in the Khan lamp concerns the substrate material itself. Sapphire, fused quartz and the other materials used in the Khan patent for transparent or translucent substrates are brittle and difficult to process. The operation of the Khan device is also limited to a positive column discharge by the device geometry.
Others have proposed cavities in hollow metal cathodes having diameters of approximately 700 .mu.m. As early as 1959, White examined hollow cathode devices having typical diameters of 750 .mu.m formed in a variety of metals, including molybdenum and niobium. White, "New Hollow Cathode Glow Discharge", J. Appl. Phys. 30, 711 (1959). More recently, Schoenbach produced and studied hollow cathode lamps having cavities with diameters of approximately 700 .mu.m machined in molybdenum. Schoenbach et al., "Microhollow Cathode Discharges", Appl. Phys. Lett. 68, 13 (1996). The processes used to produce cavities having diameters of approximately 700 .mu.m in metals are not conducive to mass production, however. In addition, sputtering of metal walls in the cathode limits device lifetime.
Schoenbach also recognized the benefit of cavities smaller than 700 .mu.m. However, such cavities are not directly machinable in metals by known methods. Though Schoenbach reported an effective cavity of 75 .mu.m in molybendum, this cavity consisted of a machined hole on the order of 700 .mu.m forming most of the cathode, and a smaller 75 .mu.m cathode opening, thus producing a microcavity opening only at the top of the device. This arrangement would not lend itself to mass production, and it is not clear that the performance characteristics of such a two-section cathode would be similar to a true microcavity cathode having a maximum diameter from below about 500 .mu.m down to about a single micrometer.
Another concern with metal cathode devices is the formation of environmentally hazardous stable compounds when the metal reacts with a compound filler, such as xenon iodide. Such fillers are potentially interesting for their ability to produce emissions similar to the resonance emission of mercury, an environmentally hazardous compound. However, compounds such as xenon iodide tend to react and form stable hazardous compounds with metal cathodes.
Accordingly, it is an object of the present invention to provide a microdischarge lamp that solves several limitations of such conventional lamps.
A further object of the present invention is to provide a microdischarge lamp having at least one microcavity in a one-piece substrate which forms a cathode around the microcavity, where the microcavity has an aperture from about a single micrometer to approximately 400 .mu.m.
Another object of the present invention is to provide a microdischarge lamp including a microcavity in a silicon substrate which contains a conductive filler, such as gas or vapor, wherein the filler is electrically contacted by a semiconductor cathode formed in the silicon around the microcavity.
Still another object of the present invention is to provide a microdischarge lamp including a microcavity in a silicon substrate which contains a conductive filler, the filler being electrically contacted by one or more semiconductor electrodes formed in the silicon, wherein the lamp is operable as hollow cathode discharge at a pD (pressure diameter) exceeding approximately 20 Torr-mm depending on the selected ratio of the cavity length to the cavity aperture.